Facelift S3 Traction Control Frequency

Shmelll said:

What tyres do you have as standard on the S3 facelift then?

Click to expand...

Some get Hankook special Korean ditchfinders, I got dunlop sportmaxx, optional 19's seem to get more exotic rubber.

mikemod said:

Same here, don't drive like a hooligan, managed 30+ years in fast cars without going through the hedge.

Just been out for petrol so tried the same thing with the TC off, round a corner get to the hill and hit the throttle, this time there was a minimal bit of wheel spin and the car took off with no issues so the TC seems to be intervening when there is no need.

Maybe it detects that I have Hankook tyres on and throttles back..

Click to expand...

4K said:

Very interesting. From the sounds of it then, the RS3 just has software changes in the ECU to make it 'feel' more planted on all 4's, but essentially it still is a Haldex system. Very nice deception for another 10K.

Click to expand...

This is the thing...it's still haldex,but with alterations to the software,and the software,whether in the Haldex,or TC or ECU is key to how the torque is managed,and sent in the best direction.

Audi's system is designed not to chuck you into the bushes,and for that characteristic to be difficult to override,which is generally a very good thing.
There are better software systems to be had,including those that use torque based TC,as opposed to wheel braking systems,but that's expensive.

The fundamental "problem" with the Haldex system is where the power/torque take-off comes from,and thats before any other differentials,meaning that in most circumstances(other than where slip is occurring),the best torque split can't exceed 50/50.
On top of that,you have the software governing how fast the clutch is loaded or preloaded,and how that relates to throttle angle etc etc.

The HPA controllers are excellent,but yet another aftermarket device.

Rob2k68 said:

I get the TC thing going round corners or hitting a slippy white line or manhole cover with one wheel but without much throttle input (forgetting any power increase for a moment) getting the slippy car icon flickering on the dash in the first few gears on a slippy road can't be right.

I have a vague understanding of what's going on front to rear axle drive wise with the prop to the rear diff spinning at the same rpm as the front drive shafts then the Haldex pump engages a clutch in order to engage drive to the rear diff. Can anyone (GSB?) expand on that ? What's the 'normal' power bias front to rear and what circumstances effect this ? What's the maximum rear bias ?

Click to expand...

Of course.

Torque distrubution is from 100% front - 0% rear, to 0% front - 100% rear. This is because torque distribution is not a constant value, and is entirely dependant on the amount of grip available at the wheels and what the control system is trying to do at the time, which is far from constant and changes by the second. Torque in the drive system is a reaction against the tyres grip on the road, so normal is a little hard to define, if theres no reaction from the tyre because it's on ice/airborn, then there's no real torque. This confusion isn't really surprising since for the most part torque split ratios are a marketing tool more than a real chassis dynamics concern.

There's a huge amount of mis-information and misunderstanding around this board on all wheel drive on the A3/S3 and how it works. I think it might be a good idea to get a few myths out in the open and dispel them once and for all;

Things like a maximum 90:10 torque split, the 'fact' that haldex only reacts to front wheelspin, and the "not as good as as 'proper' quattro" line are all very suspect, and for the most part, wrong.

Let's explore why...


Reading these pages it strikes me as odd that there's not an idiots guide to Quattro here somewhere. There's clearly a lot of knowledge here, but there's not a lot of explanation of what's actually happening in the oily bits of these cars, and what's so 'good' about it.

So, Perhaps an explanation of exactly what's going on will help.

In traditional all wheel drive, the engines effort emerges at the output from the gearbox and is sent to a centre differential. This works exactly the same way as the differential in a two wheel drive car, except instead of splitting power left and right to a wheel, it splits it forwards and backwards to the front and rear axles.

The front and rear axles also contain a differential, to allow the Left and Right wheels to turn at different speeds for cornering. So, you have three differentials fitted, two prop shafts, and four driveshafts. This is a lot of extra iron to lug around if you don't really need four wheel drive all the time. However, both the marketing men and we consumers love all this all wheel drive stuff, so we're all converts to the 'benefits' of all wheel drive, and have decided we definitely need it without really understanding the reasons why beyond a vague recollection that 30 odd years ago Audi wiped the floor with the competition on the world rally stage, and has been dining out on the back of the mythical Ur-Quattro ever since. There are many reasons it's good, but there is an equally compelling list of reasons we'd be better off without it. We'll touch in this later, but for now, let's look at what's actually going on under these cars...

Now, it is my experience that most people don't know how a differential works, and to understand four wheel drive, you need to understand differentials. It's not hard to understand and it's nothing to be embarrassed about it you don't know, but if this includes you then before we go any further, watch this film. Its retro, it's comical, and it's the best explanation of how a differential works you'll ever see;



So, now you're an expert in differentials, specifically so called 'open' differentials. With an open diff such as the one on that video, there's a distinct disadvantage. If one wheel has no grip, then that wheel spins away all the power and the car doesn't move. In a traditional four wheel drive system, one wheel with no grip will also spin away all the power through that axle, and because the affected axle is hooked up to the centre diff, that also spins away to all the power to the free side, and the four wheel drive is useless. All four wheels must have contact with something grippy for the system to work. There's many ways to get around this problem, but to save us getting bogged down in Torsen and other limited slip technologies that'll just confuse everyone, we'll stick with the traditional version, which is the locking centre diff.

Any off-roader worthy of the name had a locking centre diff in the good old days (the years BWEABD - Before Widespread Electronics And ****** Diesels), and this was a simple, light, and very effective way to dramatically improve your off-roaders chances on the trail by preventing the centre differential from being a differential. It locked the front and rear output shafts together to stop it from 'differentiating'. If you sat in an old Charles Spencer King Range Rover though, you couldn't help but notice this very stern warning about its use;



The problem with locking the front axle to the rear axle is a phenomenon called 'wind-up'.

This happens when the car turns, or even if the tyres are even slightly different diameters due to uneven wear. The difference in the speeds of the wheels relative to each other when turning corners creates enourmous stress on the transmission components, that can only be relieved when one or more wheels 'skips' by breaking traction and skidding slightly.

Fine when you're off road on slippy surfaces or ploughing a muddy field when the tyres can slip and slide with wild abandon, but on grippy Tarmac? Something has to give, and more often than not it's an expensive part of your oily bits. So, a centre diff is absolutely, no questions, do-not-pass-go-and-collect-£200, essential.

Or is it?

Take a look at a haldex equipped car. You can search for hours but you won't find a centre diff. It's extremely obvious in its absence. In fact, mechanically speaking, (and ignoring the haldex coupling for a moment) the front diff is locked to the rear diff just like an old Land Rover with centre diff lock engaged.

To avoid the perilous wind up, a clutch is fitted to the rear prop shaft to allow a bit of slip, and so relieve the wind up issue. This is the haldex coupling, and it's actually pretty simple. It's a clutch, same as the one the more adventurous, suave, sophisticated, attractive and sexually appealing members here operate with their left feet. Those of you with S&Tonic boxes will just have to use your imagination here, and try to remember what swapping your own cogs involved... ;-)

This haldex clutch can vary its amount of slip just like the one under your left foot, from no drive whatsoever, through slipping and transmitting a bit of torque, right up to fully locked up and transmitting everything you can throw at it.

This being the case, it can theoretically send 100% of the engines torque to the rear. It's only theoretical of course since you'd need to pulling a wheelie for this to actually happen. In practise, just like a traditional four wheel drive system it transmits torque all round.

When driving, the haldex clutch is partially engaged most of the time, with a bit of slip allowed to avoid transmission wind-up and save you from big repair bills. The computer driving the system looks at many variables and sensors like steering angle, throttle position, yaw, lateral and longitudinal accelleration, and compares these values on a map to see how much clutch should be engaged at any given time. It reduces clutch engagement to lower fuel consumption and stresses when the rear axle isn't needed, and when you give it the beans, it engages the clutch to full locking, allowing the rear axle to do its job of driving the car without wasting power.

In most situations though, the system is in full time four wheel drive mode. It can't transmit full torque to the rear axle all the time like 'proper' Quattro, but it does transmit full torque when it needs too.

So where do all these myths and misconceptions come from?

Well, the very first of these haldex type 'on demand' systems on which most bias and preconceptions are based, used a viscous coupling or some other reactive driveline system. It was a passive system that reacted to things that had already happened and diverted some power to the rear axle in very defined circumstances. The front wheels had to be going faster then than the rear wheels, i.e. The front wheels would be losing grip and maybe even spinning. This was advanced in later versions by using this difference in speed to trigger and power a mechanical pump to force the clutch plates together and drive the rear axle. As soon as the front and rear wheels were going the same speed again, the pump stopped and the clutch disengaged. This system was unable to drive the rear wheels under any other circumstances. The manufacturers jumped on this system though, offering 4wd 'soft roaders' to gullible fashion victims the world over. Technically they were indeed 4wd, but they were rubbish, and a traditional 4wd drive would run rings round them. The Honda CRV is a laughable and lamentable example of this trend. When tested the rear axle couldn't actually muster enough torque to move the car, since it was built by marketing men with a four wheel drive system consisting of lightweight (and therefore fuel efficient) driveshafts crafted from spaghetti, and a rear diff made of cheese. This is marketing cynicism at its worst, but hopefully Audi are above that, right?

Later, haldex made the pump electrically powered and lost the viscous element, meaning they could have active control of the clutch and engage it 'before' the front axle lost grip. It was no longer reactive, and could be controlled and engaged before something happened to require rear drive. Now we're on the 5th generation of this system and it's totally integrated into the cars dynamic control systems. It talks to traction control, esp, suspension systems and all manner of other stuff, making it more effective, less intrusive, and more importantly, vastly tunable to different applications, whether they be mud plugging or in our case, a more sporting intent. The software that drives the system is now key to how the system feels in the car. The Quattro selections in the MMI will define just how fast the clutch is ordered to engage in 'spirited' driving, or reduce the clutch engagement for more comfort and economy. When manoeuvring or at extreme steering lock you may find the rear drive line is isolated entirely, to avoid the wind up and odd hopping sensations as the tyres slip.

But I can't get the back end out. Why not?

No, you can't. This is because whilst the haldex equipped car can vary the torque transfer to its rear wheels, just like a 'proper' Quattro can, it can't reduce the torque transfer to the front. It's fixed, mechanically linked, and always fully engaged. It is this single fact that gives us the predominantly front drive feel to these cars. Well, that, and the fact that they're basically front drive cars to start with. If you really wanted to play at being an oversteer hero though, you should have rubbed a few brain cells together and realised that four wheel drive, of any type, is not really suited to such showboating. Try an ultimate driving machine, or better yet, an MX5.

The Focus RS can do it!

Yes yes, but it's faking it. The focus RS uses the system first employed in the tarts 4x4 of choice, the Range Rover Evoque. Instead of a clutch coupling feeding into a rear differential and then to the wheels, the focus doesn't have a rear differential at all. Instead the axle is effectively solid, but has a haldex style clutch for each wheel. This way it can allow the clutches to slip a bit so the car can still go round corners, or, for its party piece (and proof that even very sensible engineers can have a laugh if they get bored), it can completely disengage one wheel and send all the torque to one side of the axle. This, understandably, creates some very wierd chassis dynamics that would under any other circumstances be a hazard to health and well being, but is in fact a hooligans wet dream. It's still predominantly front wheel drive though, just like your A3/S3.

Why can't we have a 'proper' quattro system?

I'm sure Audi's marketing men would prefer it too, but the fact is that the full fat system is three things that the haldex system is not;

1/ heavy. A third differential, an extra prop shaft, a lot of extra hard points to bolt it all too. The extra weight is detrimental to handling, fuel economy and performance. It wouldn't be worth fitting it.
2/ bulky. Just where are you going to put this stuff? You need to get a transfer box in somewhere, plus an independent front diff and an extra prop shaft in the space normally occupied by the engine. Packaging is a bit tricky.
3/ expensive. Ok, maybe this isn't an issue, we all bought Audi's after all, but the fact is developing such a system for small cars isn't cheap, and the prices Audi would need to charge might be a bit prohibitive.

Besides, this misses the final, ultimate quandary. The fact no-one here will ever acknowledge...

We don't need four wheel drive. It's just a marketing ploy to make tarted up golfs more appealing. Front drive chassis tech can easily cope with 300hp these days and the benefits of a lighter car without all this ironmongery whirling around underneath it would far outweigh the benefits of driving the rear wheels a bit. However, without Quattro, Audi's history would look very different, and it would by now be about as desirable as Opel. Mercedes and BMW have shown that worldwide perception is this;

Rear drive = fun, premium, sporting, expensive and desirable.
Front drive = boring, cheap, nasty, and distinctly blue collar.
All wheel drive, see rear drive.

So, If Audi wants to play with the other germans, it needs to get some torque to the back axle. We should be grateful that what is really a bit of a token effort to do this is actually very effective on the road...

Now, what was the question again?

GSB said:

Of course.

Torque distrubution is from 100% front - 0% rear, to 0% front - 100% rear. This is because torque distribution is not a constant value, and is entirely dependant on the amount of grip available at the wheels and what the control system is trying to do at the time, which is far from constant and changes by the second. Torque in the drive system is a reaction against the tyres grip on the road, so normal is a little hard to define, if theres no reaction from the tyre because it's on ice/airborn, then there's no real torque. This confusion isn't really surprising since for the most part torque split ratios are a marketing tool more than a real chassis dynamics concern.

There's a huge amount of mis-information and misunderstanding around this board on all wheel drive on the A3/S3 and how it works. I think it might be a good idea to get a few myths out in the open and dispel them once and for all;

Things like a maximum 90:10 torque split, the 'fact' that haldex only reacts to front wheelspin, and the "not as good as as 'proper' quattro" line are all very suspect, and for the most part, wrong.

Let's explore why...


Reading these pages it strikes me as odd that there's not an idiots guide to Quattro here somewhere. There's clearly a lot of knowledge here, but there's not a lot of explanation of what's actually happening in the oily bits of these cars, and what's so 'good' about it.

So, Perhaps an explanation of exactly what's going on will help.

In traditional all wheel drive, the engines effort emerges at the output from the gearbox and is sent to a centre differential. This works exactly the same way as the differential in a two wheel drive car, except instead of splitting power left and right to a wheel, it splits it forwards and backwards to the front and rear axles.

The front and rear axles also contain a differential, to allow the Left and Right wheels to turn at different speeds for cornering. So, you have three differentials fitted, two prop shafts, and four driveshafts. This is a lot of extra iron to lug around if you don't really need four wheel drive all the time. However, both the marketing men and we consumers love all this all wheel drive stuff, so we're all converts to the 'benefits' of all wheel drive, and have decided we definitely need it without really understanding the reasons why beyond a vague recollection that 30 odd years ago Audi wiped the floor with the competition on the world rally stage, and has been dining out on the back of the mythical Ur-Quattro ever since. There are many reasons it's good, but there is an equally compelling list of reasons we'd be better off without it. We'll touch in this later, but for now, let's look at what's actually going on under these cars...

Now, it is my experience that most people don't know how a differential works, and to understand four wheel drive, you need to understand differentials. It's not hard to understand and it's nothing to be embarrassed about it you don't know, but if this includes you then before we go any further, watch this film. Its retro, it's comical, and it's the best explanation of how a differential works you'll ever see;



So, now you're an expert in differentials, specifically so called 'open' differentials. With an open diff such as the one on that video, there's a distinct disadvantage. If one wheel has no grip, then that wheel spins away all the power and the car doesn't move. In a traditional four wheel drive system, one wheel with no grip will also spin away all the power through that axle, and because the affected axle is hooked up to the centre diff, that also spins away to all the power to the free side, and the four wheel drive is useless. All four wheels must have contact with something grippy for the system to work. There's many ways to get around this problem, but to save us getting bogged down in Torsen and other limited slip technologies that'll just confuse everyone, we'll stick with the traditional version, which is the locking centre diff.

Any off-roader worthy of the name had a locking centre diff in the good old days (the years BWEABD - Before Widespread Electronics And ****** Diesels), and this was a simple, light, and very effective way to dramatically improve your off-roaders chances on the trail by preventing the centre differential from being a differential. It locked the front and rear output shafts together to stop it from 'differentiating'. If you sat in an old Charles Spencer King Range Rover though, you couldn't help but notice this very stern warning about its use;

View attachment 107840

The problem with locking the front axle to the rear axle is a phenomenon called 'wind-up'.

This happens when the car turns, or even if the tyres are even slightly different diameters due to uneven wear. The difference in the speeds of the wheels relative to each other when turning corners creates enourmous stress on the transmission components, that can only be relieved when one or more wheels 'skips' by breaking traction and skidding slightly.
View attachment 107841
Fine when you're off road on slippy surfaces or ploughing a muddy field when the tyres can slip and slide with wild abandon, but on grippy Tarmac? Something has to give, and more often than not it's an expensive part of your oily bits. So, a centre diff is absolutely, no questions, do-not-pass-go-and-collect-£200, essential.

Or is it?

Take a look at a haldex equipped car. You can search for hours but you won't find a centre diff. It's extremely obvious in its absence. In fact, mechanically speaking, (and ignoring the haldex coupling for a moment) the front diff is locked to the rear diff just like an old Land Rover with centre diff lock engaged.

To avoid the perilous wind up, a clutch is fitted to the rear prop shaft to allow a bit of slip, and so relieve the wind up issue. This is the haldex coupling, and it's actually pretty simple. It's a clutch, same as the one the more adventurous, suave, sophisticated, attractive and sexually appealing members here operate with their left feet. Those of you with S&Tonic boxes will just have to use your imagination here, and try to remember what swapping your own cogs involved... ;-)

This haldex clutch can vary its amount of slip just like the one under your left foot, from no drive whatsoever, through slipping and transmitting a bit of torque, right up to fully locked up and transmitting everything you can throw at it.

This being the case, it can theoretically send 100% of the engines torque to the rear. It's only theoretical of course since you'd need to pulling a wheelie for this to actually happen. In practise, just like a traditional four wheel drive system it transmits torque all round.

When driving, the haldex clutch is partially engaged most of the time, with a bit of slip allowed to avoid transmission wind-up and save you from big repair bills. The computer driving the system looks at many variables and sensors like steering angle, throttle position, yaw, lateral and longitudinal accelleration, and compares these values on a map to see how much clutch should be engaged at any given time. It reduces clutch engagement to lower fuel consumption and stresses when the rear axle isn't needed, and when you give it the beans, it engages the clutch to full locking, allowing the rear axle to do its job of driving the car without wasting power.

In most situations though, the system is in full time four wheel drive mode. It can't transmit full torque to the rear axle all the time like 'proper' Quattro, but it does transmit full torque when it needs too.

So where do all these myths and misconceptions come from?

Well, the very first of these haldex type 'on demand' systems on which most bias and preconceptions are based, used a viscous coupling or some other reactive driveline system. It was a passive system that reacted to things that had already happened and diverted some power to the rear axle in very defined circumstances. The front wheels had to be going faster then than the rear wheels, i.e. The front wheels would be losing grip and maybe even spinning. This was advanced in later versions by using this difference in speed to trigger and power a mechanical pump to force the clutch plates together and drive the rear axle. As soon as the front and rear wheels were going the same speed again, the pump stopped and the clutch disengaged. This system was unable to drive the rear wheels under any other circumstances. The manufacturers jumped on this system though, offering 4wd 'soft roaders' to gullible fashion victims the world over. Technically they were indeed 4wd, but they were rubbish, and a traditional 4wd drive would run rings round them. The Honda CRV is a laughable and lamentable example of this trend. When tested the rear axle couldn't actually muster enough torque to move the car, since it was built by marketing men with a four wheel drive system consisting of lightweight (and therefore fuel efficient) driveshafts crafted from spaghetti, and a rear diff made of cheese. This is marketing cynicism at its worst, but hopefully Audi are above that, right?

Later, haldex made the pump electrically powered and lost the viscous element, meaning they could have active control of the clutch and engage it 'before' the front axle lost grip. It was no longer reactive, and could be controlled and engaged before something happened to require rear drive. Now we're on the 5th generation of this system and it's totally integrated into the cars dynamic control systems. It talks to traction control, esp, suspension systems and all manner of other stuff, making it more effective, less intrusive, and more importantly, vastly tunable to different applications, whether they be mud plugging or in our case, a more sporting intent. The software that drives the system is now key to how the system feels in the car. The Quattro selections in the MMI will define just how fast the clutch is ordered to engage in 'spirited' driving, or reduce the clutch engagement for more comfort and economy. When manoeuvring or at extreme steering lock you may find the rear drive line is isolated entirely, to avoid the wind up and odd hopping sensations as the tyres slip.

But I can't get the back end out. Why not?

No, you can't. This is because whilst the haldex equipped car can vary the torque transfer to its rear wheels, just like a 'proper' Quattro can, it can't reduce the torque transfer to the front. It's fixed, mechanically linked, and always fully engaged. It is this single fact that gives us the predominantly front drive feel to these cars. Well, that, and the fact that they're basically front drive cars to start with. If you really wanted to play at being an oversteer hero though, you should have rubbed a few brain cells together and realised that four wheel drive, of any type, is not really suited to such showboating. Try an ultimate driving machine, or better yet, an MX5.

The Focus RS can do it!

Yes yes, but it's faking it. The focus RS uses the system first employed in the tarts 4x4 of choice, the Range Rover Evoque. Instead of a clutch coupling feeding into a rear differential and then to the wheels, the focus doesn't have a rear differential at all. Instead the axle is effectively solid, but has a haldex style clutch for each wheel. This way it can allow the clutches to slip a bit so the car can still go round corners, or, for its party piece (and proof that even very sensible engineers can have a laugh if they get bored), it can completely disengage one wheel and send all the torque to one side of the axle. This, understandably, creates some very wierd chassis dynamics that would under any other circumstances be a hazard to health and well being, but is in fact a hooligans wet dream. It's still predominantly front wheel drive though, just like your A3/S3.

Why can't we have a 'proper' quattro system?

I'm sure Audi's marketing men would prefer it too, but the fact is that the full fat system is three things that the haldex system is not;

1/ heavy. A third differential, an extra prop shaft, a lot of extra hard points to bolt it all too. The extra weight is detrimental to handling, fuel economy and performance. It wouldn't be worth fitting it.
2/ bulky. Just where are you going to put this stuff? You need to get a transfer box in somewhere, plus an independent front diff and an extra prop shaft in the space normally occupied by the engine. Packaging is a bit tricky.
3/ expensive. Ok, maybe this isn't an issue, we all bought Audi's after all, but the fact is developing such a system for small cars isn't cheap, and the prices Audi would need to charge might be a bit prohibitive.

Besides, this misses the final, ultimate quandary. The fact no-one here will ever acknowledge...

We don't need four wheel drive. It's just a marketing ploy to make tarted up golfs more appealing. Front drive chassis tech can easily cope with 300hp these days and the benefits of a lighter car without all this ironmongery whirling around underneath it would far outweigh the benefits of driving the rear wheels a bit. However, without Quattro, Audi's history would look very different, and it would by now be about as desirable as Opel. Mercedes and BMW have shown that worldwide perception is this;

Rear drive = fun, premium, sporting, expensive and desirable.
Front drive = boring, cheap, nasty, and distinctly blue collar.
All wheel drive, see rear drive.

So, If Audi wants to play with the other germans, it needs to get some torque to the back axle. We should be grateful that what is really a bit of a token effort to do this is actually very effective on the road...

Now, what was the question again?

Click to expand...

Excellent informative post GSB thanks for taking time to do this.

Helps a lot with my crude understanding i'm beginning to think that my issues are not down to any oily bits but to a sensor or some binary code - will know more on Tuesday.

Defective sensors might be an issue. I was considering this myself whilst pondering just how bad a set of Hankooks can be.

Most sensors involved in this system will be solid state or Hall effect proximity sensors, so no moving parts and very high reliability factors, but like all processes, if you put defective signals in you'll get a poor output.

GSB said:

Of course.

Torque distrubution is from 100% front - 0% rear, to 0% front - 100% rear. This is because torque distribution is not a constant value, and is entirely dependant on the amount of grip available at the wheels and what the control system is trying to do at the time, which is far from constant and changes by the second. Torque in the drive system is a reaction against the tyres grip on the road, so normal is a little hard to define, if theres no reaction from the tyre because it's on ice/airborn, then there's no real torque. This confusion isn't really surprising since for the most part torque split ratios are a marketing tool more than a real chassis dynamics concern.

There's a huge amount of mis-information and misunderstanding around this board on all wheel drive on the A3/S3 and how it works. I think it might be a good idea to get a few myths out in the open and dispel them once and for all;

Things like a maximum 90:10 torque split, the 'fact' that haldex only reacts to front wheelspin, and the "not as good as as 'proper' quattro" line are all very suspect, and for the most part, wrong.

Let's explore why...


Reading these pages it strikes me as odd that there's not an idiots guide to Quattro here somewhere. There's clearly a lot of knowledge here, but there's not a lot of explanation of what's actually happening in the oily bits of these cars, and what's so 'good' about it.

So, Perhaps an explanation of exactly what's going on will help.

In traditional all wheel drive, the engines effort emerges at the output from the gearbox and is sent to a centre differential. This works exactly the same way as the differential in a two wheel drive car, except instead of splitting power left and right to a wheel, it splits it forwards and backwards to the front and rear axles.

The front and rear axles also contain a differential, to allow the Left and Right wheels to turn at different speeds for cornering. So, you have three differentials fitted, two prop shafts, and four driveshafts. This is a lot of extra iron to lug around if you don't really need four wheel drive all the time. However, both the marketing men and we consumers love all this all wheel drive stuff, so we're all converts to the 'benefits' of all wheel drive, and have decided we definitely need it without really understanding the reasons why beyond a vague recollection that 30 odd years ago Audi wiped the floor with the competition on the world rally stage, and has been dining out on the back of the mythical Ur-Quattro ever since. There are many reasons it's good, but there is an equally compelling list of reasons we'd be better off without it. We'll touch in this later, but for now, let's look at what's actually going on under these cars...

Now, it is my experience that most people don't know how a differential works, and to understand four wheel drive, you need to understand differentials. It's not hard to understand and it's nothing to be embarrassed about it you don't know, but if this includes you then before we go any further, watch this film. Its retro, it's comical, and it's the best explanation of how a differential works you'll ever see;



So, now you're an expert in differentials, specifically so called 'open' differentials. With an open diff such as the one on that video, there's a distinct disadvantage. If one wheel has no grip, then that wheel spins away all the power and the car doesn't move. In a traditional four wheel drive system, one wheel with no grip will also spin away all the power through that axle, and because the affected axle is hooked up to the centre diff, that also spins away to all the power to the free side, and the four wheel drive is useless. All four wheels must have contact with something grippy for the system to work. There's many ways to get around this problem, but to save us getting bogged down in Torsen and other limited slip technologies that'll just confuse everyone, we'll stick with the traditional version, which is the locking centre diff.

Any off-roader worthy of the name had a locking centre diff in the good old days (the years BWEABD - Before Widespread Electronics And ****** Diesels), and this was a simple, light, and very effective way to dramatically improve your off-roaders chances on the trail by preventing the centre differential from being a differential. It locked the front and rear output shafts together to stop it from 'differentiating'. If you sat in an old Charles Spencer King Range Rover though, you couldn't help but notice this very stern warning about its use;

View attachment 107840

The problem with locking the front axle to the rear axle is a phenomenon called 'wind-up'.

This happens when the car turns, or even if the tyres are even slightly different diameters due to uneven wear. The difference in the speeds of the wheels relative to each other when turning corners creates enourmous stress on the transmission components, that can only be relieved when one or more wheels 'skips' by breaking traction and skidding slightly.
View attachment 107841
Fine when you're off road on slippy surfaces or ploughing a muddy field when the tyres can slip and slide with wild abandon, but on grippy Tarmac? Something has to give, and more often than not it's an expensive part of your oily bits. So, a centre diff is absolutely, no questions, do-not-pass-go-and-collect-£200, essential.

Or is it?

Take a look at a haldex equipped car. You can search for hours but you won't find a centre diff. It's extremely obvious in its absence. In fact, mechanically speaking, (and ignoring the haldex coupling for a moment) the front diff is locked to the rear diff just like an old Land Rover with centre diff lock engaged.

To avoid the perilous wind up, a clutch is fitted to the rear prop shaft to allow a bit of slip, and so relieve the wind up issue. This is the haldex coupling, and it's actually pretty simple. It's a clutch, same as the one the more adventurous, suave, sophisticated, attractive and sexually appealing members here operate with their left feet. Those of you with S&Tonic boxes will just have to use your imagination here, and try to remember what swapping your own cogs involved... ;-)

This haldex clutch can vary its amount of slip just like the one under your left foot, from no drive whatsoever, through slipping and transmitting a bit of torque, right up to fully locked up and transmitting everything you can throw at it.

This being the case, it can theoretically send 100% of the engines torque to the rear. It's only theoretical of course since you'd need to pulling a wheelie for this to actually happen. In practise, just like a traditional four wheel drive system it transmits torque all round.

When driving, the haldex clutch is partially engaged most of the time, with a bit of slip allowed to avoid transmission wind-up and save you from big repair bills. The computer driving the system looks at many variables and sensors like steering angle, throttle position, yaw, lateral and longitudinal accelleration, and compares these values on a map to see how much clutch should be engaged at any given time. It reduces clutch engagement to lower fuel consumption and stresses when the rear axle isn't needed, and when you give it the beans, it engages the clutch to full locking, allowing the rear axle to do its job of driving the car without wasting power.

In most situations though, the system is in full time four wheel drive mode. It can't transmit full torque to the rear axle all the time like 'proper' Quattro, but it does transmit full torque when it needs too.

So where do all these myths and misconceptions come from?

Well, the very first of these haldex type 'on demand' systems on which most bias and preconceptions are based, used a viscous coupling or some other reactive driveline system. It was a passive system that reacted to things that had already happened and diverted some power to the rear axle in very defined circumstances. The front wheels had to be going faster then than the rear wheels, i.e. The front wheels would be losing grip and maybe even spinning. This was advanced in later versions by using this difference in speed to trigger and power a mechanical pump to force the clutch plates together and drive the rear axle. As soon as the front and rear wheels were going the same speed again, the pump stopped and the clutch disengaged. This system was unable to drive the rear wheels under any other circumstances. The manufacturers jumped on this system though, offering 4wd 'soft roaders' to gullible fashion victims the world over. Technically they were indeed 4wd, but they were rubbish, and a traditional 4wd drive would run rings round them. The Honda CRV is a laughable and lamentable example of this trend. When tested the rear axle couldn't actually muster enough torque to move the car, since it was built by marketing men with a four wheel drive system consisting of lightweight (and therefore fuel efficient) driveshafts crafted from spaghetti, and a rear diff made of cheese. This is marketing cynicism at its worst, but hopefully Audi are above that, right?

Later, haldex made the pump electrically powered and lost the viscous element, meaning they could have active control of the clutch and engage it 'before' the front axle lost grip. It was no longer reactive, and could be controlled and engaged before something happened to require rear drive. Now we're on the 5th generation of this system and it's totally integrated into the cars dynamic control systems. It talks to traction control, esp, suspension systems and all manner of other stuff, making it more effective, less intrusive, and more importantly, vastly tunable to different applications, whether they be mud plugging or in our case, a more sporting intent. The software that drives the system is now key to how the system feels in the car. The Quattro selections in the MMI will define just how fast the clutch is ordered to engage in 'spirited' driving, or reduce the clutch engagement for more comfort and economy. When manoeuvring or at extreme steering lock you may find the rear drive line is isolated entirely, to avoid the wind up and odd hopping sensations as the tyres slip.

But I can't get the back end out. Why not?

No, you can't. This is because whilst the haldex equipped car can vary the torque transfer to its rear wheels, just like a 'proper' Quattro can, it can't reduce the torque transfer to the front. It's fixed, mechanically linked, and always fully engaged. It is this single fact that gives us the predominantly front drive feel to these cars. Well, that, and the fact that they're basically front drive cars to start with. If you really wanted to play at being an oversteer hero though, you should have rubbed a few brain cells together and realised that four wheel drive, of any type, is not really suited to such showboating. Try an ultimate driving machine, or better yet, an MX5.

The Focus RS can do it!

Yes yes, but it's faking it. The focus RS uses the system first employed in the tarts 4x4 of choice, the Range Rover Evoque. Instead of a clutch coupling feeding into a rear differential and then to the wheels, the focus doesn't have a rear differential at all. Instead the axle is effectively solid, but has a haldex style clutch for each wheel. This way it can allow the clutches to slip a bit so the car can still go round corners, or, for its party piece (and proof that even very sensible engineers can have a laugh if they get bored), it can completely disengage one wheel and send all the torque to one side of the axle. This, understandably, creates some very wierd chassis dynamics that would under any other circumstances be a hazard to health and well being, but is in fact a hooligans wet dream. It's still predominantly front wheel drive though, just like your A3/S3.

Why can't we have a 'proper' quattro system?

I'm sure Audi's marketing men would prefer it too, but the fact is that the full fat system is three things that the haldex system is not;

1/ heavy. A third differential, an extra prop shaft, a lot of extra hard points to bolt it all too. The extra weight is detrimental to handling, fuel economy and performance. It wouldn't be worth fitting it.
2/ bulky. Just where are you going to put this stuff? You need to get a transfer box in somewhere, plus an independent front diff and an extra prop shaft in the space normally occupied by the engine. Packaging is a bit tricky.
3/ expensive. Ok, maybe this isn't an issue, we all bought Audi's after all, but the fact is developing such a system for small cars isn't cheap, and the prices Audi would need to charge might be a bit prohibitive.

Besides, this misses the final, ultimate quandary. The fact no-one here will ever acknowledge...

We don't need four wheel drive. It's just a marketing ploy to make tarted up golfs more appealing. Front drive chassis tech can easily cope with 300hp these days and the benefits of a lighter car without all this ironmongery whirling around underneath it would far outweigh the benefits of driving the rear wheels a bit. However, without Quattro, Audi's history would look very different, and it would by now be about as desirable as Opel. Mercedes and BMW have shown that worldwide perception is this;

Rear drive = fun, premium, sporting, expensive and desirable.
Front drive = boring, cheap, nasty, and distinctly blue collar.
All wheel drive, see rear drive.

So, If Audi wants to play with the other germans, it needs to get some torque to the back axle. We should be grateful that what is really a bit of a token effort to do this is actually very effective on the road...

Now, what was the question again?

Click to expand...

As always Grant very interesting and entertaining....Great write up...

GSB said:

Of course.

Torque distrubution is from 100% front - 0% rear, to 0% front - 100% rear. This is because torque distribution is not a constant value, and is entirely dependant on the amount of grip available at the wheels and what the control system is trying to do at the time, which is far from constant and changes by the second. Torque in the drive system is a reaction against the tyres grip on the road, so normal is a little hard to define, if theres no reaction from the tyre because it's on ice/airborn, then there's no real torque. This confusion isn't really surprising since for the most part torque split ratios are a marketing tool more than a real chassis dynamics concern.

There's a huge amount of mis-information and misunderstanding around this board on all wheel drive on the A3/S3 and how it works. I think it might be a good idea to get a few myths out in the open and dispel them once and for all;

Things like a maximum 90:10 torque split, the 'fact' that haldex only reacts to front wheelspin, and the "not as good as as 'proper' quattro" line are all very suspect, and for the most part, wrong.

Let's explore why...


Reading these pages it strikes me as odd that there's not an idiots guide to Quattro here somewhere. There's clearly a lot of knowledge here, but there's not a lot of explanation of what's actually happening in the oily bits of these cars, and what's so 'good' about it.

So, Perhaps an explanation of exactly what's going on will help.

In traditional all wheel drive, the engines effort emerges at the output from the gearbox and is sent to a centre differential. This works exactly the same way as the differential in a two wheel drive car, except instead of splitting power left and right to a wheel, it splits it forwards and backwards to the front and rear axles.

The front and rear axles also contain a differential, to allow the Left and Right wheels to turn at different speeds for cornering. So, you have three differentials fitted, two prop shafts, and four driveshafts. This is a lot of extra iron to lug around if you don't really need four wheel drive all the time. However, both the marketing men and we consumers love all this all wheel drive stuff, so we're all converts to the 'benefits' of all wheel drive, and have decided we definitely need it without really understanding the reasons why beyond a vague recollection that 30 odd years ago Audi wiped the floor with the competition on the world rally stage, and has been dining out on the back of the mythical Ur-Quattro ever since. There are many reasons it's good, but there is an equally compelling list of reasons we'd be better off without it. We'll touch in this later, but for now, let's look at what's actually going on under these cars...

Now, it is my experience that most people don't know how a differential works, and to understand four wheel drive, you need to understand differentials. It's not hard to understand and it's nothing to be embarrassed about it you don't know, but if this includes you then before we go any further, watch this film. Its retro, it's comical, and it's the best explanation of how a differential works you'll ever see;



So, now you're an expert in differentials, specifically so called 'open' differentials. With an open diff such as the one on that video, there's a distinct disadvantage. If one wheel has no grip, then that wheel spins away all the power and the car doesn't move. In a traditional four wheel drive system, one wheel with no grip will also spin away all the power through that axle, and because the affected axle is hooked up to the centre diff, that also spins away to all the power to the free side, and the four wheel drive is useless. All four wheels must have contact with something grippy for the system to work. There's many ways to get around this problem, but to save us getting bogged down in Torsen and other limited slip technologies that'll just confuse everyone, we'll stick with the traditional version, which is the locking centre diff.

Any off-roader worthy of the name had a locking centre diff in the good old days (the years BWEABD - Before Widespread Electronics And ****** Diesels), and this was a simple, light, and very effective way to dramatically improve your off-roaders chances on the trail by preventing the centre differential from being a differential. It locked the front and rear output shafts together to stop it from 'differentiating'. If you sat in an old Charles Spencer King Range Rover though, you couldn't help but notice this very stern warning about its use;

View attachment 107840

The problem with locking the front axle to the rear axle is a phenomenon called 'wind-up'.

This happens when the car turns, or even if the tyres are even slightly different diameters due to uneven wear. The difference in the speeds of the wheels relative to each other when turning corners creates enourmous stress on the transmission components, that can only be relieved when one or more wheels 'skips' by breaking traction and skidding slightly.
View attachment 107841
Fine when you're off road on slippy surfaces or ploughing a muddy field when the tyres can slip and slide with wild abandon, but on grippy Tarmac? Something has to give, and more often than not it's an expensive part of your oily bits. So, a centre diff is absolutely, no questions, do-not-pass-go-and-collect-£200, essential.

Or is it?

Take a look at a haldex equipped car. You can search for hours but you won't find a centre diff. It's extremely obvious in its absence. In fact, mechanically speaking, (and ignoring the haldex coupling for a moment) the front diff is locked to the rear diff just like an old Land Rover with centre diff lock engaged.

To avoid the perilous wind up, a clutch is fitted to the rear prop shaft to allow a bit of slip, and so relieve the wind up issue. This is the haldex coupling, and it's actually pretty simple. It's a clutch, same as the one the more adventurous, suave, sophisticated, attractive and sexually appealing members here operate with their left feet. Those of you with S&Tonic boxes will just have to use your imagination here, and try to remember what swapping your own cogs involved... ;-)

This haldex clutch can vary its amount of slip just like the one under your left foot, from no drive whatsoever, through slipping and transmitting a bit of torque, right up to fully locked up and transmitting everything you can throw at it.

This being the case, it can theoretically send 100% of the engines torque to the rear. It's only theoretical of course since you'd need to pulling a wheelie for this to actually happen. In practise, just like a traditional four wheel drive system it transmits torque all round.

When driving, the haldex clutch is partially engaged most of the time, with a bit of slip allowed to avoid transmission wind-up and save you from big repair bills. The computer driving the system looks at many variables and sensors like steering angle, throttle position, yaw, lateral and longitudinal accelleration, and compares these values on a map to see how much clutch should be engaged at any given time. It reduces clutch engagement to lower fuel consumption and stresses when the rear axle isn't needed, and when you give it the beans, it engages the clutch to full locking, allowing the rear axle to do its job of driving the car without wasting power.

In most situations though, the system is in full time four wheel drive mode. It can't transmit full torque to the rear axle all the time like 'proper' Quattro, but it does transmit full torque when it needs too.

So where do all these myths and misconceptions come from?

Well, the very first of these haldex type 'on demand' systems on which most bias and preconceptions are based, used a viscous coupling or some other reactive driveline system. It was a passive system that reacted to things that had already happened and diverted some power to the rear axle in very defined circumstances. The front wheels had to be going faster then than the rear wheels, i.e. The front wheels would be losing grip and maybe even spinning. This was advanced in later versions by using this difference in speed to trigger and power a mechanical pump to force the clutch plates together and drive the rear axle. As soon as the front and rear wheels were going the same speed again, the pump stopped and the clutch disengaged. This system was unable to drive the rear wheels under any other circumstances. The manufacturers jumped on this system though, offering 4wd 'soft roaders' to gullible fashion victims the world over. Technically they were indeed 4wd, but they were rubbish, and a traditional 4wd drive would run rings round them. The Honda CRV is a laughable and lamentable example of this trend. When tested the rear axle couldn't actually muster enough torque to move the car, since it was built by marketing men with a four wheel drive system consisting of lightweight (and therefore fuel efficient) driveshafts crafted from spaghetti, and a rear diff made of cheese. This is marketing cynicism at its worst, but hopefully Audi are above that, right?

Later, haldex made the pump electrically powered and lost the viscous element, meaning they could have active control of the clutch and engage it 'before' the front axle lost grip. It was no longer reactive, and could be controlled and engaged before something happened to require rear drive. Now we're on the 5th generation of this system and it's totally integrated into the cars dynamic control systems. It talks to traction control, esp, suspension systems and all manner of other stuff, making it more effective, less intrusive, and more importantly, vastly tunable to different applications, whether they be mud plugging or in our case, a more sporting intent. The software that drives the system is now key to how the system feels in the car. The Quattro selections in the MMI will define just how fast the clutch is ordered to engage in 'spirited' driving, or reduce the clutch engagement for more comfort and economy. When manoeuvring or at extreme steering lock you may find the rear drive line is isolated entirely, to avoid the wind up and odd hopping sensations as the tyres slip.

But I can't get the back end out. Why not?

No, you can't. This is because whilst the haldex equipped car can vary the torque transfer to its rear wheels, just like a 'proper' Quattro can, it can't reduce the torque transfer to the front. It's fixed, mechanically linked, and always fully engaged. It is this single fact that gives us the predominantly front drive feel to these cars. Well, that, and the fact that they're basically front drive cars to start with. If you really wanted to play at being an oversteer hero though, you should have rubbed a few brain cells together and realised that four wheel drive, of any type, is not really suited to such showboating. Try an ultimate driving machine, or better yet, an MX5.

The Focus RS can do it!

Yes yes, but it's faking it. The focus RS uses the system first employed in the tarts 4x4 of choice, the Range Rover Evoque. Instead of a clutch coupling feeding into a rear differential and then to the wheels, the focus doesn't have a rear differential at all. Instead the axle is effectively solid, but has a haldex style clutch for each wheel. This way it can allow the clutches to slip a bit so the car can still go round corners, or, for its party piece (and proof that even very sensible engineers can have a laugh if they get bored), it can completely disengage one wheel and send all the torque to one side of the axle. This, understandably, creates some very wierd chassis dynamics that would under any other circumstances be a hazard to health and well being, but is in fact a hooligans wet dream. It's still predominantly front wheel drive though, just like your A3/S3.

Why can't we have a 'proper' quattro system?

I'm sure Audi's marketing men would prefer it too, but the fact is that the full fat system is three things that the haldex system is not;

1/ heavy. A third differential, an extra prop shaft, a lot of extra hard points to bolt it all too. The extra weight is detrimental to handling, fuel economy and performance. It wouldn't be worth fitting it.
2/ bulky. Just where are you going to put this stuff? You need to get a transfer box in somewhere, plus an independent front diff and an extra prop shaft in the space normally occupied by the engine. Packaging is a bit tricky.
3/ expensive. Ok, maybe this isn't an issue, we all bought Audi's after all, but the fact is developing such a system for small cars isn't cheap, and the prices Audi would need to charge might be a bit prohibitive.

Besides, this misses the final, ultimate quandary. The fact no-one here will ever acknowledge...

We don't need four wheel drive. It's just a marketing ploy to make tarted up golfs more appealing. Front drive chassis tech can easily cope with 300hp these days and the benefits of a lighter car without all this ironmongery whirling around underneath it would far outweigh the benefits of driving the rear wheels a bit. However, without Quattro, Audi's history would look very different, and it would by now be about as desirable as Opel. Mercedes and BMW have shown that worldwide perception is this;

Rear drive = fun, premium, sporting, expensive and desirable.
Front drive = boring, cheap, nasty, and distinctly blue collar.
All wheel drive, see rear drive.

So, If Audi wants to play with the other germans, it needs to get some torque to the back axle. We should be grateful that what is really a bit of a token effort to do this is actually very effective on the road...

Now, what was the question again?

Click to expand...

Jesus Christ, and I thought my kids A-level coursework was confusing!! lol

Great stuff Grant

pburv said:

As always Grant very interesting and entertaining....Great write up...

Click to expand...

Simon L said:

Jesus Christ, and I thought my kids A-level coursework was confusing!! lol

Great stuff Grant

Click to expand...

Why thank you...

I do think we need a sticky on how that Haldex system, or rather the Borg Warner Torqtransfer system (haldex was bought out years ago) actually works. Its a source of immense confusion and misinformation here.

GSB said:

Why thank you...

I do think we need a sticky on how that Haldex system, or rather the Borg Warner Torqtransfer system (haldex was bought out years ago) actually works. Its a source of immense confusion and misinformation here.

Click to expand...

GSB said:

Of course.

Torque distrubution is from 100% front - 0% rear, to 0% front - 100% rear. This is because torque distribution is not a constant value, and is entirely dependant on the amount of grip available at the wheels and what the control system is trying to do at the time, which is far from constant and changes by the second. Torque in the drive system is a reaction against the tyres grip on the road, so normal is a little hard to define, if theres no reaction from the tyre because it's on ice/airborn, then there's no real torque. This confusion isn't really surprising since for the most part torque split ratios are a marketing tool more than a real chassis dynamics concern.

There's a huge amount of mis-information and misunderstanding around this board on all wheel drive on the A3/S3 and how it works. I think it might be a good idea to get a few myths out in the open and dispel them once and for all;

Things like a maximum 90:10 torque split, the 'fact' that haldex only reacts to front wheelspin, and the "not as good as as 'proper' quattro" line are all very suspect, and for the most part, wrong.

Let's explore why...


Reading these pages it strikes me as odd that there's not an idiots guide to Quattro here somewhere. There's clearly a lot of knowledge here, but there's not a lot of explanation of what's actually happening in the oily bits of these cars, and what's so 'good' about it.

So, Perhaps an explanation of exactly what's going on will help.

In traditional all wheel drive, the engines effort emerges at the output from the gearbox and is sent to a centre differential. This works exactly the same way as the differential in a two wheel drive car, except instead of splitting power left and right to a wheel, it splits it forwards and backwards to the front and rear axles.

The front and rear axles also contain a differential, to allow the Left and Right wheels to turn at different speeds for cornering. So, you have three differentials fitted, two prop shafts, and four driveshafts. This is a lot of extra iron to lug around if you don't really need four wheel drive all the time. However, both the marketing men and we consumers love all this all wheel drive stuff, so we're all converts to the 'benefits' of all wheel drive, and have decided we definitely need it without really understanding the reasons why beyond a vague recollection that 30 odd years ago Audi wiped the floor with the competition on the world rally stage, and has been dining out on the back of the mythical Ur-Quattro ever since. There are many reasons it's good, but there is an equally compelling list of reasons we'd be better off without it. We'll touch in this later, but for now, let's look at what's actually going on under these cars...

Now, it is my experience that most people don't know how a differential works, and to understand four wheel drive, you need to understand differentials. It's not hard to understand and it's nothing to be embarrassed about it you don't know, but if this includes you then before we go any further, watch this film. Its retro, it's comical, and it's the best explanation of how a differential works you'll ever see;



So, now you're an expert in differentials, specifically so called 'open' differentials. With an open diff such as the one on that video, there's a distinct disadvantage. If one wheel has no grip, then that wheel spins away all the power and the car doesn't move. In a traditional four wheel drive system, one wheel with no grip will also spin away all the power through that axle, and because the affected axle is hooked up to the centre diff, that also spins away to all the power to the free side, and the four wheel drive is useless. All four wheels must have contact with something grippy for the system to work. There's many ways to get around this problem, but to save us getting bogged down in Torsen and other limited slip technologies that'll just confuse everyone, we'll stick with the traditional version, which is the locking centre diff.

Any off-roader worthy of the name had a locking centre diff in the good old days (the years BWEABD - Before Widespread Electronics And ****** Diesels), and this was a simple, light, and very effective way to dramatically improve your off-roaders chances on the trail by preventing the centre differential from being a differential. It locked the front and rear output shafts together to stop it from 'differentiating'. If you sat in an old Charles Spencer King Range Rover though, you couldn't help but notice this very stern warning about its use;

View attachment 107840

The problem with locking the front axle to the rear axle is a phenomenon called 'wind-up'.

This happens when the car turns, or even if the tyres are even slightly different diameters due to uneven wear. The difference in the speeds of the wheels relative to each other when turning corners creates enourmous stress on the transmission components, that can only be relieved when one or more wheels 'skips' by breaking traction and skidding slightly.
View attachment 107841
Fine when you're off road on slippy surfaces or ploughing a muddy field when the tyres can slip and slide with wild abandon, but on grippy Tarmac? Something has to give, and more often than not it's an expensive part of your oily bits. So, a centre diff is absolutely, no questions, do-not-pass-go-and-collect-£200, essential.

Or is it?

Take a look at a haldex equipped car. You can search for hours but you won't find a centre diff. It's extremely obvious in its absence. In fact, mechanically speaking, (and ignoring the haldex coupling for a moment) the front diff is locked to the rear diff just like an old Land Rover with centre diff lock engaged.

To avoid the perilous wind up, a clutch is fitted to the rear prop shaft to allow a bit of slip, and so relieve the wind up issue. This is the haldex coupling, and it's actually pretty simple. It's a clutch, same as the one the more adventurous, suave, sophisticated, attractive and sexually appealing members here operate with their left feet. Those of you with S&Tonic boxes will just have to use your imagination here, and try to remember what swapping your own cogs involved... ;-)

This haldex clutch can vary its amount of slip just like the one under your left foot, from no drive whatsoever, through slipping and transmitting a bit of torque, right up to fully locked up and transmitting everything you can throw at it.

This being the case, it can theoretically send 100% of the engines torque to the rear. It's only theoretical of course since you'd need to pulling a wheelie for this to actually happen. In practise, just like a traditional four wheel drive system it transmits torque all round.

When driving, the haldex clutch is partially engaged most of the time, with a bit of slip allowed to avoid transmission wind-up and save you from big repair bills. The computer driving the system looks at many variables and sensors like steering angle, throttle position, yaw, lateral and longitudinal accelleration, and compares these values on a map to see how much clutch should be engaged at any given time. It reduces clutch engagement to lower fuel consumption and stresses when the rear axle isn't needed, and when you give it the beans, it engages the clutch to full locking, allowing the rear axle to do its job of driving the car without wasting power.

In most situations though, the system is in full time four wheel drive mode. It can't transmit full torque to the rear axle all the time like 'proper' Quattro, but it does transmit full torque when it needs too.

So where do all these myths and misconceptions come from?

Well, the very first of these haldex type 'on demand' systems on which most bias and preconceptions are based, used a viscous coupling or some other reactive driveline system. It was a passive system that reacted to things that had already happened and diverted some power to the rear axle in very defined circumstances. The front wheels had to be going faster then than the rear wheels, i.e. The front wheels would be losing grip and maybe even spinning. This was advanced in later versions by using this difference in speed to trigger and power a mechanical pump to force the clutch plates together and drive the rear axle. As soon as the front and rear wheels were going the same speed again, the pump stopped and the clutch disengaged. This system was unable to drive the rear wheels under any other circumstances. The manufacturers jumped on this system though, offering 4wd 'soft roaders' to gullible fashion victims the world over. Technically they were indeed 4wd, but they were rubbish, and a traditional 4wd drive would run rings round them. The Honda CRV is a laughable and lamentable example of this trend. When tested the rear axle couldn't actually muster enough torque to move the car, since it was built by marketing men with a four wheel drive system consisting of lightweight (and therefore fuel efficient) driveshafts crafted from spaghetti, and a rear diff made of cheese. This is marketing cynicism at its worst, but hopefully Audi are above that, right?

Later, haldex made the pump electrically powered and lost the viscous element, meaning they could have active control of the clutch and engage it 'before' the front axle lost grip. It was no longer reactive, and could be controlled and engaged before something happened to require rear drive. Now we're on the 5th generation of this system and it's totally integrated into the cars dynamic control systems. It talks to traction control, esp, suspension systems and all manner of other stuff, making it more effective, less intrusive, and more importantly, vastly tunable to different applications, whether they be mud plugging or in our case, a more sporting intent. The software that drives the system is now key to how the system feels in the car. The Quattro selections in the MMI will define just how fast the clutch is ordered to engage in 'spirited' driving, or reduce the clutch engagement for more comfort and economy. When manoeuvring or at extreme steering lock you may find the rear drive line is isolated entirely, to avoid the wind up and odd hopping sensations as the tyres slip.

But I can't get the back end out. Why not?

No, you can't. This is because whilst the haldex equipped car can vary the torque transfer to its rear wheels, just like a 'proper' Quattro can, it can't reduce the torque transfer to the front. It's fixed, mechanically linked, and always fully engaged. It is this single fact that gives us the predominantly front drive feel to these cars. Well, that, and the fact that they're basically front drive cars to start with. If you really wanted to play at being an oversteer hero though, you should have rubbed a few brain cells together and realised that four wheel drive, of any type, is not really suited to such showboating. Try an ultimate driving machine, or better yet, an MX5.

The Focus RS can do it!

Yes yes, but it's faking it. The focus RS uses the system first employed in the tarts 4x4 of choice, the Range Rover Evoque. Instead of a clutch coupling feeding into a rear differential and then to the wheels, the focus doesn't have a rear differential at all. Instead the axle is effectively solid, but has a haldex style clutch for each wheel. This way it can allow the clutches to slip a bit so the car can still go round corners, or, for its party piece (and proof that even very sensible engineers can have a laugh if they get bored), it can completely disengage one wheel and send all the torque to one side of the axle. This, understandably, creates some very wierd chassis dynamics that would under any other circumstances be a hazard to health and well being, but is in fact a hooligans wet dream. It's still predominantly front wheel drive though, just like your A3/S3.

Why can't we have a 'proper' quattro system?

I'm sure Audi's marketing men would prefer it too, but the fact is that the full fat system is three things that the haldex system is not;

1/ heavy. A third differential, an extra prop shaft, a lot of extra hard points to bolt it all too. The extra weight is detrimental to handling, fuel economy and performance. It wouldn't be worth fitting it.
2/ bulky. Just where are you going to put this stuff? You need to get a transfer box in somewhere, plus an independent front diff and an extra prop shaft in the space normally occupied by the engine. Packaging is a bit tricky.
3/ expensive. Ok, maybe this isn't an issue, we all bought Audi's after all, but the fact is developing such a system for small cars isn't cheap, and the prices Audi would need to charge might be a bit prohibitive.

Besides, this misses the final, ultimate quandary. The fact no-one here will ever acknowledge...

We don't need four wheel drive. It's just a marketing ploy to make tarted up golfs more appealing. Front drive chassis tech can easily cope with 300hp these days and the benefits of a lighter car without all this ironmongery whirling around underneath it would far outweigh the benefits of driving the rear wheels a bit. However, without Quattro, Audi's history would look very different, and it would by now be about as desirable as Opel. Mercedes and BMW have shown that worldwide perception is this;

Rear drive = fun, premium, sporting, expensive and desirable.
Front drive = boring, cheap, nasty, and distinctly blue collar.
All wheel drive, see rear drive.

So, If Audi wants to play with the other germans, it needs to get some torque to the back axle. We should be grateful that what is really a bit of a token effort to do this is actually very effective on the road...

Now, what was the question again?

Click to expand...

Hilariously entertaining and informative post. You remind me of my old History teacher. Enjoyed the video, and your S&T jibe. You've definitely cleared up a lot of misunderstandings that have perpetuated around the forums for quite a while, for me at least. Did you ever consider applying for a job on Top Gear? You might have been able to save its demise.

I take it these new wave of '****** Diesels' such as Kia Sportage, Qashqai and the like are similar then to the Honda CR-V of the day with crumbling rear diffs?

4K said:

Hilariously entertaining and informative post. You remind me of my old History teacher. Enjoyed the video, and your S&T jibe. You've definitely cleared up a lot of misunderstandings that have perpetuated around the forums for quite a while, for me at least. Did you ever consider applying for a job on Top Gear? You might have been able to save its demise.

I take it these new wave of '****** Diesels' such as Kia Sportage, Qashqai and the like are similar then to the Honda CR-V of the day with crumbling rear diffs?

Click to expand...

No, I never got into journalism, a friend of mine did though, and he's been editor and writer of several well known U.K. car magazines. I know it's not for me though. I could never handle the ordeal of familiarising myself with an endless succession of £100k+ sports cars, or keep up the demanding pace of endless first class flights and all expenses paid boozy junkets all over the world for manufacturers to kiss my backside and provide me with cars, clothes, watches, gifts for the wife and hot & cold running hookers.

I've never taken the likes of a Juke or a Kuga or a sportage off road, so I couldn't really tell you if they suffer from the same marketing department chocolate rear drivetrains that Honda fitted, but I wouldn't be surprised. Properly implemented though, theres no reason these systems shouldn't be very very effective. The LR freelander uses Haldex, and it's a fine off roader. Not as capable as a discovery obviously, but it can hack the rough stuff.

The evoque uses the more advanced GKN twinster system which is even more capable and is the one system that really shows the future for AWD. Don't get your hopes up though, Focus RS aside, it's a future that's light years away from the world of 80's rally stages, and looks very very boring. In pursuit of ever higher fuel economy whilst retaining the AWD go anywhere USP, the evoque goes 100% front wheel drive over about 20mph. It uses the same two clutches in the rear axle to mimic the differential function, plus a third clutch to disengage the propshaft entirely in the power take off box at the front. This additional complexity removes the drag from the rear driveline almost entirely.

Audi have come up with a bizarrre hybrid of bits, called it quattro ultra, and you'll now find it underneath most Quattro versions of the previously all conquering and 'better' centre diff equipped A4. Even the A4 allroad is now 100% front wheel drive almost all the time. The system uses a variant of the Haldex unit we all know and love, but it's now on the back of the gearbox. In addition they've put a dog clutch on one of the rear driveshafts. When in front drive mode, which will be most of the time, the Haldex stops driving the propshaft from one end, and the dog cluctch stops the wheels spinning the propshaft from the other. All to reduce rotating drag and add an extra mpg or two.

Since this system is earmarked for future S & RS models, we MBQ drivers might find that instead of driving the inferior version of Quattro, we'll find ourselves actually driving the full time Quattro swan song...

May as well close this forum down now, GSB has won.


Sent from my iPhone using Tapatalk

Just out of interest it looks like the Haldex controller ECU went from revision D and S/W version 7076 on 15 plate cars to revision H and S/W version 7083 on 65 plate cars.

Rob2k68 said:

Just out of interest it looks like the Haldex controller ECU went from revision D and S/W version 7076 on 15 plate cars to revision H and S/W version 7083 on 65 plate cars.

Click to expand...

Be interested to know what changes they made, since mine is 65 plate.

4K said:

Be interested to know what changes they made, since mine is 65 plate.

Click to expand...

Yeah - me too mate. Anybody 'in the know' out there ?

D is not available to buy anymore so H has to be backwards compatible.

Those suffering from excessive and unwarranted operation of traction control, especially in new cars, please see this thread and provide as much info as you can.

http://www.audi-sport.net/xf/threads/s3-traction-control-too-sensitive-poll.301767/

GSB in short are you saying the Haldex-V found on the 8V A3,S3, and RS3 can provide torque splitting at greatest 60/40? or 54.5 / 45.5? There's significant amount of torque sent to the rear, but the car still holds a front wheel bias.

unless the front of the car goes airborne or front wheels lock up, then you will see 0/100 ?

Not really sure about the question, and if you think I have the capability to say anything other than my name "in short" you're probably mistaken!

What I'm saying is that torque splits are mostly marketing speak used to fill brochures and get punters into showrooms.

Torque at each wheel is a function of the amount of grip being used at the wheel in question and the acceleration of the vehicle. The amount of grip is massively dependant upon the amount of mass the wheel is carrying, and this varies hugely with cornering forces and weight transfer under braking and acceleration. Thus is is not possible to say 45/55, 0/100 etc, since it's basically marketing rubbish. No one talks about left/right talk splits when we're cornering, and yet the torque at left and right wheels varies hugely.

It is possible, and fairly common, to design a centre differential that biases torque transfer in one direction over the other, but the S3 doesn't have one. Under severe acceleration the system in our cars behaves like a traditional awd system with a locked centre differential. In this scenario, in straight line acceleration, the front and rear wheels will rotate at the same speed, but the amount of torque being successfully transmitted by each wheel will differ. The fronts will naturally lose traction first due to the wieght transfer, but since the transmission is connected equally to front and rear, the rears will keep pushing at the same rate of acceleration and the loss of traction at the front won't be felt. When the system is in this mode, there is no defined or deliberate bias toward front or rear as the two are mechanically linked, but the weight shift though will mean the front tyres are sufficiently unloaded that they have very limited grip on the road surface, and if the torque at the front wheels exceeds their grip the lions share of the work will be coming from the rear tyres.

Under almost all other driving conditions, the amount of torque transferred to the rear axle is limited by the Haldex coupling to save fuel and avoid transmission wind up. This is when the system is more obviously front biased. The system will be locked until the torque from the engine exceeds the torque limit at the coupling, when it will slip. The front axle has the same mechanical link between engine and wheels as a front drive car. This link cannot be limited, altered or turned down, only the rear axle has this capability. The inherent front bias with this Haldex system originates only in this mechanical layout. Put a haldex type coupling into a 911 for instance, and you have a predominantly rear drive car that is able to shunt power forwards when desired. No one ever complains that those cars aren't 'proper' AWD, do they...

Here's a paper on AWD systems you might find useful reading;